Thread: LIKE query on indexes

hi,
i have btree index on a text type field. i want see rows which starts with certain characters on that field. so i write a query like this:

SELECT * FROM mytable WHERE myfield LIKE 'john%'

since this condition is from start of the field, query planner should use index to find such elements but explain command shows me it will do a sequential scan.

is this lack of a feature or i am wrong somewhere?

On Tue, 2006-02-21 at 09:57, Ibrahim Tekin wrote:
> hi,
> i have btree index on a text type field. i want see rows which starts
> with certain characters on that field. so i write a query like this:
>
> SELECT * FROM mytable WHERE myfield LIKE 'john%'
>
> since this condition is from start of the field, query planner should
> use index to find such elements but explain command shows me it will
> do a sequential scan.
>
> is this lack of a feature or i am wrong somewhere?

This is an artifact of how PostgreSQL handles locales other than ASCII.

If you want such a query to use an index, you need to back up your
database, and re-initdb with --locale=C as an argument.  Note that you
then will NOT get locale specific matching and sorting.

Scott Marlowe wrote:
> On Tue, 2006-02-21 at 09:57, Ibrahim Tekin wrote:
> > hi,
> > i have btree index on a text type field. i want see rows which starts
> > with certain characters on that field. so i write a query like this:
> >
> > SELECT * FROM mytable WHERE myfield LIKE 'john%'
> >
> > since this condition is from start of the field, query planner should
> > use index to find such elements but explain command shows me it will
> > do a sequential scan.
> >
> > is this lack of a feature or i am wrong somewhere?
>
> This is an artifact of how PostgreSQL handles locales other than ASCII.
>
> If you want such a query to use an index, you need to back up your
> database, and re-initdb with --locale=C as an argument.

... or you can choose to create an index with the text_pattern_ops
operator class, which would be used in a LIKE constraint regardless of
locale.

http://www.postgresql.org/docs/8.1/static/indexes-opclass.html

--
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

On Tue, 2006-02-21 at 10:34, Alvaro Herrera wrote:
> Scott Marlowe wrote:
> > On Tue, 2006-02-21 at 09:57, Ibrahim Tekin wrote:
> > > hi,
> > > i have btree index on a text type field. i want see rows which starts
> > > with certain characters on that field. so i write a query like this:
> > >
> > > SELECT * FROM mytable WHERE myfield LIKE 'john%'
> > >
> > > since this condition is from start of the field, query planner should
> > > use index to find such elements but explain command shows me it will
> > > do a sequential scan.
> > >
> > > is this lack of a feature or i am wrong somewhere?
> >
> > This is an artifact of how PostgreSQL handles locales other than ASCII.
> >
> > If you want such a query to use an index, you need to back up your
> > database, and re-initdb with --locale=C as an argument.
>
> ... or you can choose to create an index with the text_pattern_ops
> operator class, which would be used in a LIKE constraint regardless of
> locale.
>
> http://www.postgresql.org/docs/8.1/static/indexes-opclass.html

Good point.  I tend to view the world from the perspective of the 7.4
and before user...

On Tue, Feb 21, 2006 at 05:57:12PM +0200, Ibrahim Tekin wrote:
> i have btree index on a text type field. i want see rows which starts with
> certain characters on that field. so i write a query like this:
>     SELECT * FROM mytable WHERE myfield LIKE 'john%'
> since this condition is from start of the field, query planner should use
> index to find such elements but explain command shows me it will do a
> sequential scan.
> is this lack of a feature or i am wrong somewhere?

Is the query fast enough? How big is your table? What does explain
analyze select tell you?

Cheers,
mark

--
mark@mielke.cc / markm@ncf.ca / markm@nortel.com     __________________________
.  .  _  ._  . .   .__    .  . ._. .__ .   . . .__  | Neighbourhood Coder
|\/| |_| |_| |/    |_     |\/|  |  |_  |   |/  |_   |
|  | | | | \ | \   |__ .  |  | .|. |__ |__ | \ |__  | Ottawa, Ontario, Canada

  One ring to rule them all, one ring to find them, one ring to bring them all
                       and in the darkness bind them...

                           http://mark.mielke.cc/

my database encoding is unicode.
i have two table, one is 3.64gb on hdd and has 2.2 million records. it takes 140 secs to run on my AMD Turion 64 M 800MHz/1GB laptop.
second table is 1.2gb, 220000 records, and takes 56 secs to run.

explain says 'Seq Scan on mytable, ..'

this trick did the job.
thanks.

Hi,

hi,

i ran a query with ILIKE but it doesn't use the index.

but i tried following method, and it worked. there is 3 extra lower() overhead but i don't think it will effect the performance.

CREATE INDEX index_name ON mytable (lower(column) varchar_pattern_ops);

SELECT * FROM mytable WHERE lower(column) LIKE lower('beginswith%')

if insert operations are high in database. you use only this index to search case sensitive.

say you want this:
SELECT * FROM mytable WHERE column LIKE 'beGinsWith%'

write this:
SELECT * FROM mytable WHERE lower(column) LIKE lower('beGinsWith%') AND column LIKE 'beGinsWith%'

than query planner will search on index, than scan the resulting bitmap heap.